Methods of coating rubbing surfaces and elements comprising such surfaces

ABSTRACT

TO INCREASE THE WEAR RESISTANCE OF A ROTARY PISTON ENEGINE CASING ENVELOPE THE INNER SURFACE OF THE ENVELOPE IS COATED BY A KEYING FILM AND THEN A RESISTANT SUB-LAYER OF AN ALLOY OF STEEL AND NICKEL ALUMINIDES, THEN BY A THIN WEAR-RESISTANT LAYER OF NICKEL OR COBALT. THE SUB-LAYER IS FORMED BY FLAME-SPRAYING SIMULTANEOUSLY 90% TO 70% BY VOLUME CARBON STEEL WITH 10% TO 30% OF A MIXUTRE OF EQUAL PARTS OF NICKEL AND ALUMINIUM POWDERS.

Dec. 12, GRQSSEAU. METHODS OF COATING RUBBING SURFACES AND ELEMENTS COMPRISING S SURFA Filed Dec. 196

INVENTOR ALBERT GROSS EAU BY ATTORNEYS United States Patent 3,705,818 METHODS OF COATING RUBBING SURFACES AND ELEMENTS COMPRISING SUCH SURFACES Albert Grosseau, Chaville, France, assignor to Socrete Anonyme Automobiles Citroen, Paris, France Filed Dec. 31, 1969, Ser. No. 889,569 Claims priority, appliclegtion France, Dec. 31, 1968,

int. c1. C23c 7700,- B44d 1/16 US. Cl. 117-71 M 7 Claims ABSTRACT OF THE DISCLOSURE The present invention relates to methods of coating rubbing surfaces, especially internal surfaces of crankcasing envelopes, for rotary piston engine, constituted of light alloy and to the elements comprising such surfaces.

It is known that, on the operation of a rotary piston engine, the inner surface of the envelope of the crankcasing is subjected, on the side of the rotary piston and its segments, to considerable contact pressures and to a considerable friction, and it is already known to fix, on this surface, coatings constituted by materials which improve the rubbing and possess high resistance to wear, and which are applied by methods known as electrolytic and/ or chemical means.

It is also known to deposit first of all on the said inner surface of the crank-casing envelope an adherent metallic sub-layer, hard and of suitable thickness for obtaining the required resistance to contact pressures. This metallic sublayer is generally obtained by spraying, by means of an oxyacetylene flame or a plasma flame. There is then applied on this sub-layer, by electrolytic and/or chemical means, a thin wear-resistant coating.

It is an object of the invention, particularly, to render the said methods and elements such that they respond better than up to the present to the various exigencies of practice, especially as regards resistance to wear of the rubbing surfaceand reduction of the coefiicient of friction of the member, such as a rotary piston, which cooperates with the above-said element.

According to the invention, there is provided a method of coating rubbing surfaces, especially internal surfaces of crank-casing envelopes, for rotary piston engines, constituted of light alloy, according to which there is first deposited, by flame projection, on said surfaces previously machined to dimensions in the neighbourhood of the final dimensions, an adherent metallic sub-layer, hard and of considerable thickness for obtaining the required resistance to contract pressures, then, after a slight correction approximating very closely the final dimensions, there is applied on this sub-layer, by electrolytic and/or chemical means, a thin wear-resistant coating on which it then sufiices to effect a slight finishing to obtain the final dimension and surface condition desired, characterized in that to obtain the metallic sub-layer there is simulice taneously projected, by means of at least two guns, on the one hand carbon steel in a proportion by volume of the order of 90% to and on the other hand in complementary proportion by volume of the order of 10 to 30% a mixture in substantially equal parts of powdered nickel and aluminium.

According to another aspect of the invention, there is provided a rubbing surface element, especially a crankcasing envelope, of light alloy, for a rotary piston engine, of which the inner surface constitutes the said rubbing surface, said element being adapted so that the abovesaid rubbing surface is covered by a metallic sub-layer and of suitable thickness for obtaining the required resistance to contact pressures and the said sub-layer being covered with a thin wear-resistant coating and characterized in that the metallic sub-layer is constituted by an alloy of carbon steel and a compound of nickel aluminides NiAl and Ni Al of which the proportions by volume may vary from substantially to 70% for steel and, complementarily, from substantially 10% to 30% for the compound of nickel aluminides.

The thickness of the metallic sub-layer is generally comprised between 0.20 and 0.80 mm.

The invention consists, apart from these principal features, in certain other features which are employed preferably at the same time and which will be more specifically described below.

In order that the invention may be more fully understood, a preferred embodiment of the method and element according to the invention, are described below purely by way of illustrative but non-limiting example, and with reference to the accompanying drawing, in which:

FIG. 1 shows, in transverse section, one embodiment of a crank-case envelope made according to the invention; and

FIG. 2 shows, on a larger scale, a partial cross-section of the coating of the envelope.

Having an envelope 1 for a rotary piston engine crankcasing constituted of light alloy, for example of aluminium alloy, of which the transverse section is in the form of a trochoid with two lobes, and proposing to effect a wear-resistant coating on the inner surface 2 of the envelope 1, the procedure is as follows.

The inner surface 2 is machined to dimensions in the neighbourhood of the final dimensions. Then there is deposited firstly, by projection in an oxy-acetylene flame or a plasma flame, onto the inner surface 2, a metallic sub-layer 3, adherent hard and of a thickness 5 suited to obtaining the required resistance to contact pressures. According to the invention, the materials are sprayed simultaneously by at least two guns. The said materials are constituted by steel containing, by weight, from 0.10% to 0.60% of carbon, and by a mixture of powdered nickel and aluminium combining to form nickel aluminides (NiAl, Ni Al) which are alloyed with the carbon steel and give a product having, in addition to its resistance properties to contact pressures, improved properties of wear resistance and of rubbing which render it suitable to make up, to a certain extent, the thin wear resistant coating in the event of deterioration of the latter. After a slight correction, approaching very closely the final dimensions, there is applied on this layer 3 by chemical means such as electrolytic means a thin wearresistant coating 4 on which it then suflices to effect a slight finishing to obtain the final dimension and surface condition desired.

Before proceding with the spraying properly so-called of the sub-layer 3, the envelope 2 is pre-heated so as to bring it and keep it, at a temperature in the neighborhood of 180 C. Then there is effected, on the inner surface 2, a sanding operation, that is to say that a jet of powdered abrasive such as, for example, of silicon carbide or of corundum, is violently projected against the said surface. The rugosity of the surface 2 is thus increased, which is favourable to the adhesion of the sublayer 3. To increase further this adhesion, there is constituted on the surface 2, by oxy-acetylene flame or by plasma flame, a thin binding film 6, formed either of nickel aluminides (NiAl and Ni Al) obtained by spraying separately the mixture of nickel and aluminum powders, or of molybdenum. There is advantageously given to this thin film which covers the whole of the surface 2, a thickness of 0.02 to 0.05 mm. This projection of molten metal on the surface 2, also called metallisation, is generally effected by means of a oxy-acetylene flame gun, supplied with a wire of the metal to be sprayed, or by means of a plasma flame gun, fed by a wire or by a mixture of powders of the metals to be projected.

Next, there is effected the spraying properly so-called of the sub-layer 3 onto the binding film 6. To this end, there is preferably used, simultaneously, at least two spray-guns supplied, on one hand, by carbon steel, and on the other hand, by a mixture of aluminium and nickel powders, and adjusted to project the molten metals at the same point of the binder film.

Means are provided to ensure a relative movement between the envelope 1 and the assembly of spray-guns so as to cause the whole of the surface to be covered to pass before the one or more guns and to ensure thus a substantially uniform deposit over the whole surface.

In the simultaneous projection by at least two guns, the discharge rates of metal projected from each gun are adjusted in the ratio of the desired proportions for the sublayer 3. These proportions may vary by volume from 90% to 70% of steel contained by weight from 0.10% to 0.60% carbon and, complementarily, from 10% to 30% of nickel and aluminium powders mixed in substantially equal parts. There are effected as many passes as is necessary to obtain a sub-layer 3 of uniform thickness 5 comprised between 0.20 and 0.80 mm. approximately. This thickness confers good resistance to contact pressures developed in the course of operation.

The sub-layer 3, obtained rapidly by metallisation is finally porous, which favours the attachment of the wearresistant layer subsequently deposited, on the said sublayer, by chemical means such as electrolytic means.

After cooling the envelope to ambient temperature which causes the appearance of a prestressing in the sublayer 3 by reason of the different coefficients of expansion for the envelope 1 and the sub-layer 3, the said sub-layer is corrected before receiving the thin wear-resistant layer 4.

This wear resistant layer 4 is advantageously constituted by a deposit of nickel or of cobalt, favourable to the sliding of the piston segments. The thickness of the coating 4 varies between 0.02 and 0.1 mm. Although the deposits by chemical means are effected in relatively slow manner, the slight thickness of the coating renders the duration of the operation acceptable. The deposit constituting the wear-resistant layer 4 may be made to include particles of material insoluble in the chemical bath such as the electrolytic bath, used for obtaining the deposit. These particles are of variable dimensions, comprised preferably between 0.02a (microns) and 400 and are selected so as to confer on the wear-resistant layer 4 supplementary properties of hardness and/or of friction.

Finally, a slight finishing of the wear-resistant coating enables the achievement of the final dimensions and surfa e condition desired.

The coating thus obtained has numerous advantages.

It resists wear very well whilst facilitating sliding of the rotary piston segments in contact therewith, the said segments being constituted of grey pig iron preferably.

In addition, the metallic sub-layer 3 ensures a hard and solid seating for the coating 4 which resists prefectly contact pressures and thus enables said coating 4 to last much longer than if it had itself to sustain these stresses alone. In addition, by effecting the spraying of the sublayer at a temperature of the order of 180 C. neighbouring those of the operating conditions, there are reduced precisely those stresses which could appear in the sublayer, on operation, as result of differences between the coefiicients of expansion of the envelope and of the sublayer.

The production of a coating of this character is economical, due to the method proposed and, in consequence the manufacture of the envelope is relatively cheap. In fact, the sub-layer 3, in spite of its thickness, is obtained rapidly by spraying, which renders the cost of the operation acceptable, although it is practically excluded, by reason of a high cost price due to the slowness of the operations, to obtain such a thickness by electrolytic and/or chemical means. On the other hand, the coating 4, resisting wear well and favourable to the sliding of the cast iron segments, may be obtained at a reasonable cost, due to the fact that it is of slight thickness, by electrolytic and/0r chemical means.

Moreover, the composition of the metallic sub-layer 3 is such that in the case of the deterioration of the thin wear-resistant coating 4 the sliding of the segments of cast iron on the said layer 3 remains satisfactory and the coefficient of friction of the said segments on the sublayer 3 is slight. The method according to the invention responds therefore well to the object which it is designed to achieve and enables the obtaining in a simple, eco nomical and rapid manner of a rubbing surface coating, especially for rotary piston engine crank-casing envelopes, which is highly resistant to wear.

As is self-evident, and as emerges besides already from the preceding description, the invention is in no way limited to those of its methods of application, nor to those of its methods of production of its various parts, which have been more particularly contemplated; it embraces, on

the contrary, all variations, especially those in which the rotary piston engine crank-casing envelope allows a different transverse section from that contemplated, namely trochoidal with two lobes, in the particular example described previously.

What I claim is:

1. A method of coating the internal surfaces of light al- 10y crankcase envelopes, for rotary piston engines, comprising depositing by flame spraying, on the said surfaces, previously machined to approximately the final dimensions, an adherent metallic sub-layer which is hard and resistant to contact pressures, then applying on this sublayer, by chemical means, a wear-resistant coating wherein, to obtain said metallic sub-layer, there is simultaneously projected, by means of at least two spray guns, carbon steel in a proportion by volume of the order of to 70% and in the complementary proportion by volume of the order of 10% to 30% a mixture in substantially equal parts of nickel and aluminium powders.

2. A method according to claim 1, wherein the chemical means used to apply the wear-resistant coating is electrolytic means.

3. A method according to claim 1, wherein the carbon steel has a carbon content comprised between 0.10 and 0.60% by weight.

4. A method according to claim 1 wherein the thin Wear-resistant coating is constituted by nickel or cobalt.

5. A method according to claim 4, wherein the thin wear-resistant coating comprises particles of material insoluble in the bath used for obtaining the said coating, the said particles having a diameter of 0.02;; to 400 1.

6. A method of coating inner surfaces of crank-casing envelopes, for rotary piston engines constituted of light alloy, according to claim 1, wherein the envelope is brought to a temperature of about 180 C., at the moment of projection of the sub-layer, so as to generate, on cooling, a prestressing in this sub-layer.

7. A method of coating inner surfaces of crank-casing envelopes, for rotary piston engines, constituted of light alloy, as claimed in claim 6, wherein there is interposed, between the inner surface of the envelope and the sub-layer, a thin film of molybdenum or of nickel aluminides to improve the attachment of the sub-layer.

References Cited UNITED STATES PATENTS ALFRED L. LEAVITT, Primary Examiner C. K. WEIFFENBACH, Assistant Examiner US. Cl. X.R. 

